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tion and reaction and that the absence of an after-efFect 

 under normal conditions when the material is stimulated 

 in vacuum may not be regarded as a proof that the 

 stimulus has not been perceived; there are indeed three 

 possibihties: 



1. That neither can a stimulus be perceived nor a 

 movement be induced in a vacuum. 



2. The stimulus is perceived, but the reaction cannot 

 take place, or the disposition towards it cannot arise. 



3. The stimulus cannot be perceived, although the 

 reaction can take place. Correns admits that he has not 

 been sufBciently successful in determining the efFect of 

 oxygen on perception and reaction separately, so that he 

 cannot décide between the above three cases. 



I hâve discussed in détail Correns' paper because it 

 is by far the most important on this subject; but never- 

 theless in some respects his methods of investigation and 

 his conclusions are open to criticism. The method of 

 exhausting seems to me to be distinctly objectionable, 

 because various changes of condition may occur in the 

 plant which are wrongly ascribed to the absence of oxygen, 

 but which really hâve quite another cause. The compa- 

 risons which Correns draws between geotropism and 

 phototropism dépend on the one hand, on experiments 

 in which the seedhngs were stimulated from 6 to 12 hours 

 in a horizontal position, on the other hand they were 

 continuously illuminated unilaterally by day-light; he does 

 not State how long this illumination lasted. It seems to me 

 by no means impossible that with a shorter period of 

 geotropic stimulation, as for example, if the geotropic 

 reaction was observed after 2 hours, he might not hâve 

 obtained any curvature at the pressure indicated by him, 

 whilst with a greater quantity of oxygen after 2 hours 

 a distinct reaction might be obtained. For the time, in 

 which, under normal conditions the reaction begins to be 



